Pozzolanic Properties of Micronized Biomass Silica in Enhancing Compressive Strength and Water Permeability of Concrete

This study presents the analysis and the benefits from using Micronized Biomass Silica (MBS) of rice husk which comprises of high content of silica. MBS was generated from controlled burning of the husk into off-white biomass silica ash and crushing the ash into micronized size. Concrete samples containing various percentages of MBS were tested for workability, compressive strength and also water permeability performances. It was found that the optimum percentage of MBS added to the concrete that lead to good performance of concrete in terms of compressive strength and water permeability was 12%. The compressive strength increased up to 43% when 12% of MBS was added to the concrete after 90 days period. Increasing the content of MBS exceeding the optimum percentage showed inferior performance of the concrete. It indicates that the pozzolanic reaction properties of MBS could improve the compressive strength and water permeability of concrete.


Introduction
Concrete is produced when cement, coarse aggregate, fine aggregate, water and admixtures are mixed thoroughly. Concrete is considered of good quality when it enables to achieve good strength and durability (Neville, 2004). Good strength is achieved when concrete can sustain maximum load imposed on it. To achieve good durability, the concrete should be able to attack from aggressive ion either externally or internally. One of the methods to produce quality concrete is by adding pozzolanic material. Pozzolanic material like fly ash, silica fume and ground granulated blastfurnace slag has been widely used in construction industry. The pozzolanic material is generally produced from by-product of wastes and thus, it has advantages in encouraging the application of waste product which is good for environmental conservation (Sampaio et al., 2000). Normally, these pozzolanic materials are functioning as cement replacement material which substitute a part of cement content. In this study, the Micronized Biomass Silica (MBS) was produced from the burning of rice husk was used as cement replacement in concrete samples. Various percentages of MBS were added to replace cement in order to determine the optimum percentage that gives the maximum performance of concrete produced. The performances considered in this paper are on the compressive strength and water permeability of the produced concrete.

Micronized Biomass Silica (MBS)
Biomass waste material used in the production of MBS was rice husks. The rice husk was taken from Bernas rice milling plant in Kedah, Malaysia. The husk was burnt under controlled in a rotary furnace at Material Laboratory of the Faculty of Civil and Environmental Engineering, UTHM (Lee et al., 2007). The husk fed was into the furnace in a continuous process through multiple distributed inlets at the furnace temperature regime of 500 °C to produce white amorphous biomass silica. This furnace is equipped with controlling opening for the air (oxygen) to come in and assist a complete burning process and thus produce low carbon content of white amorphous biomass silica ash. The perforated trays in the furnace rotated to enhance the mixing process, heat distribution and to achieve optimum burning efficiency. After one hour of burning, the white biomass silica ash falls through the perforated trays and into a collecting funnel at the downstream of the furnace. The www.ccsen characteris from Table  fixed at 50  temperatur  above 550 The averag ash was b equipment rotated for study is as ash to act e

Performance of Compressive Strength
The pozzolanic reaction occurs when MBS ash reacts with calcium hydroxide generated from the cement hydration process. The amount of pozzolanic material added is limited with the amount of hydroxide produced during the hydration. This was the reason where different percentages of MBS were added to determine the optimum amount in enhancing the compressive strength and water permeability of the concrete. Concrete sample specimens were produced based on the cement percentages replacement of MBS added which were 0%, 4%, 8%, 12%, 16%, 20% and abbreviated with M0, M4, M8, M12, M16 and M20. The mix proportions for concrete mixtures are presented in Table 4.  Table 5. C

Perform
The durab to flow a p percentage coefficient demonstra compared specimens to that of c (SiO 2 ) from silicate hy aggregate

Effects
The sulpho-aluminate and 3% of secondary phases. The calcium hydroxide, Ca(OH) 2 from cement hydration process reacts with silicon dioxide (SiO 2 ) from MBS which is known as pozzolanic reaction and the reaction process is described as in Equation 2. In this reaction, the calcium hydroxide (Ca(OH) 2 ) is transformed into secondary calcium silicate hydrate (C-S-H) gel which leads into transformation of larger pores into finer pores (Oner et al., 2005). This C-S-H gel is able to fill-up to the micro-pores and ITZ in concrete. Thus, this reaction gave a significant effect to the development of concrete strength. Finer particles sizes of MBS (24.4039 m 2 /g) as compared to cement (2.693 m 2 /g) has improved filler mechanism by densifying the cement matrix, filling the voids in concrete with hydration product, improving the bonding with aggregates and reinforced the materials like a glass fiber (Isaia et al., 2003). The physical action of pozzolanic material will contribute into denser, more homogeneous and uniform paste. Thus, it can be seen besides the pozzolanic reaction the filler effect of pozzolan material also plays role in densifying the cement matrix.
MBS possess better resistance in allowing water to permeate into the concrete and also its compressive strength as compared to that of control concrete because of pozzolanic reaction which had taken place between silicon dioxide (SiO 2 ) from MBS with calcium hydroxide (Ca(OH) 2 ) from cement hydration. The reaction leads into calcium silicate hydrate, C-S-H gel formation. The gel acts to fill up the pores in concrete and also ITZ between aggregate and cement paste. These actions lead to lower permeable characteristic of the concrete.
For samples containing percentage of MBS replacement exceeding the optimum amount, its compressive strength and water permeability will reduce. This is due to the reducing amount of cement content in concrete mixes as the replacement takes higher proportion of cementitious function. It is related to the decreasing amount of C-S-H gel which latter enable to fill up the ITZ and micropores within the matrices in concrete. Consequently, water-filled space is then created and will lower the performance of concrete.
Also, it is evident that MBS does behave like other high pozzolanic reactivity of Supplementary Cementitious Material (SCM) namely silica fume and metakolin as depicted in Table 6. It can be seen from Table 6 that MBS is in agreement with Poon et al. (2006), Wong and Abdul Razak (2005), Mazloom et al. (2004) and Zhang et al. (1996) which shows the enhancement in compressive strength of concrete as SCM was included in the concrete mixes. However, MBS concrete obtained lower compressive strength in 7 and 28 days compared to those of silica fume and metakolin in Poon et al. (2006) study for concrete with same w/c ratio. This is might be due to different strengthening mechanisms of silica fume, metakaolin and MBS in concrete. According to Poon et al. (2006), the different strengthening mechanisms for different SCM have contributed into different development of compressive strength. This is due to pozzolanic reaction which occurred from SCM has becomes as dominant mechanism at early or later age.

Conclusions
This study has indicated the potential benefit of MBS in enhancing the compressive strength and the water permeability of concrete. The pozzolanic properties of MBS satisfy the ASTM C 618-08a standard as pozzolan materials where the proportion of silica, alumina and iron oxide is 88.544%, which is exceeding the standard. It reacts with calcium hydroxide generated from cement hydration has increase the CSH gel to fill up the ITZ and micropores within the matrices in concrete. The significant findings from this study are:  Workability of fresh concrete decreased with as the content of MBS added to the concrete increased.
 The optimum percentage of MBS added to the concrete that lead to good performance of concrete in terms of compressive strength and water permeability was 12%.  The compressive strength increased up to 43% when 12% of MBS was added to the concrete after 90 days period.  Increasing the content of MBS exceeding the optimum percentage showed inferior performance of the concrete.
These findings have given good potential in adding values to normal RHA in the application of enhancing the performance of concrete.